Tank heater control system



Oct. 16, 1945. c. M. osTERHELD 2,387,178

TANK HEATR CNTROL SYSTEM Filed July 17. 1944 z'sneets-sneet 1 IN VEN TOR.

"ARA 05 riff/ffm ATTORNEY Oct. 1'6, 1945. C, M QSTERHELD Y 2,387,178

TANK HEATER CONTROL SYSTEM Filed July 17, 1944 y 2 Sheets-Sheet 2 INVETOR.

2/ ZA/Wk @swf/ffm ATTORNEY Patented @et lili, i945 TANK HEATER CONTROL SYSTEM Clark M. Osterheld, Stoughton, Wis., assignor to McGraw Electric Company, Elgin, Ill., a corporation of Delaware Application July 17, 1944, Serial No. 545,351

(Cl. 21S-39) l2 Claims.

My invention relates to electric heating and particularly to control systems for electric heaters of domestic hot water storage tanks.

Among the objects of my invention are the following: to provide a control system that shall cause energization of the electric heater of a domestic hot water tank at any time during a twenty-four hour day when the tank is less than half full of hot water, such energization continuing until all of the water in the tank is hot, and that shall cause energization of the heater after a predetermined time delay period after closing of a time-controlled switch, such energization continuing until all of the water in the tank is hot.

Other objects of my invention will either be apparent from a description of several control systems embodying my invention or will be pointed out in the course of such description and set forth particularly in the appended claims.

ln the drawings,

Figure l is a vertical, sectional view through a domestic hot water tank, with which is associated a control system embodying my invention,

Fig. 2 is a diagram of connections embodying improved system of control,

Fig. 3 is a view similar to Fig. l, and,

Fig. 4 is a diagram of connections of a modified form oi control system embodying my invention.

Referring rst of all to Fig. l of the drawings, l have there illustrated a domestic hot water tank t i which is provided with a lower cold water inlet pipe i3, an upper hot water outlet pipe i5, a layer il of heat-insulating material, and an outer casing i9. All of these details are old and well known in the art and constitute no part of my present invention.

l provide a single electric heater El, which is preferably located near to the bottom end of the tank and is preferably, although not necessarily, ci a clamp-on type, which extends around the outside of the tank il and is positioned in a tunnel member 23?.

My control system includes a lower thermallyactuable heater control switch, designated generally by the numeral 25 and including a tubular member 2l, having a closed inner end and having its open outer end portion secured in a fluidtight manner in an opening in tank il, preferably a small distance above the heater 2i. An expansion rod E9 is operatively'positioned within the tube 2'! and is adapted to engage with and be disengaged from a lug 3l of electric-insulating material secured to the free or movable end of a resilient contact arm 33, which contact arm has a contact member thereon adapted to be engaged with and disengaged from a cooperating contact member on a substantially rigid contact arm 35. The two contact arms 33 and 35 are suitably supported on blocks 3l of electric-insulating material. While l have shown a particular design of thermally-actuable switch, I do not desire to be limited thereto, but may use any other similar thermally-actuable switch known in the art. The cold water enters the tank il through the lower pipe i3, as hot water is withdrawn from the tank through the upper pipe i5, and the design construction, and adjustment of the thermally-actuable switch 25 is such that the two switch arms 33 and 35 will be in contacting engagement with each other when switch 25 is subject to cold water in the tank and will be disengaged from each other when switch 25, and particularly the tube 2l, is subject to hot water in the tank. When I speak of cold water, l refer to water the temperature of which is on the order of to 70 F., and when I speak of hot water, I refer to water the temperature of which has been raised to a value on the order oi say F., although this may be raised or lowered slightly.

l provide a thermal retarder heater control switch unit 3Q, which is preferably located in heatreceiving relation on the tank il at substantially the mid-height of the tank. For details of this thermal retarder heater control unit, reference may be had to my co-pending application, S. N. 537,941, filed May 29, 1944, and assigned to the same assignee as is the present application. While the above identified application shows only two switches, the design and construction of the switches shown in the present application is similar thereto.

The thermal retarder unit includes a rst thermal switch, comprising a bimetal bar 4|, having one end thereof fixed, and having a forked other end with pivot plates 43 liked on the side portions, which pivot plates are adapted to be engaged by the two spaced pointed inner ends of a second bimetal bar d5, having a forked inner end portion, the two bimetal bars being severally provided with longitudinally-extending slots at their adjacent'ends, in which slots is located an over center spring al, the ends of which are connected to the bimetal bars 4l and 45 just beyond the ends of the central longitudinal slots to cause movement of the outer end of bimetal bar 45 with a snap action. The outer end of bimetal bar 45 has lnsulatedly and loosely mounted thereon a contact bridgingmember I9 adapted to engage with and be disengaged from two xed contacts-5| and 53. The design, construction, and adjustment of the rst thermal heater control switch of the thermal retarder unit is such that the switch will be in closed position, that is contact bridging member 49 will be in engagement with the contacts 5| and 53, when the thermal retarder is subject to cold water in the tank, that is when enough hot water has been withdrawn from the tank to subject not only the lower thermallyactuable switch 25 to cold water, but also the thermal retarder switch unit 39, which may be located at the mid-height of the tank or above or below such fixed position. A stop member 55 is provided, which is adapted to be engaged by the free end of the outer bimetal bar l5 when the thermal retarder switch unit is subject to hot water in the tank.

I provide further a second thermal switch, comprising a part of the thermal retarder switch unit and including a first bimetal bar 5l having one end thereof mounted in fixed position and having its forked other end provided with pivot plates |53, which pivot plates are adapted to be engaged by the pointed ends of a second bimetal bar 59 having a. forked inner end. An over centerspring ti is positioned within longitudinally-extending slots in the adjacent end portions of bimetal bars 5l and 59 so that movement of the free end of the bimetal bar 59 will be effected with a snap action. The outer end of bimetal bar 59 has loosely insulatedly mounted thereon a contact bridging member 63 adapted to engage with and be disengaged from a pair of ilxed contacts 65 and 6l. The design, construction-and adjustment of the second thermal switch, comprising particularly the bimetal bars 57 and 59, are such that the contact bridging member 63 will be out of engagement with the contact members 65 and 5l when the thermal retarder switch is subject to cold water in the tank and to hot water in the tank. a-nd will be in engagement with the fixed contact members G5 and'l at an appreciably higher temperature, which may be on the order of 250 F. When the'second thermal switch is in its open position, the outer end of the second bimetal bar 59 engages a stop member.

Means for raising the temperature of the bimetal bars 51 and 59 include a heating coil 1|, which is insulatedly mounted on and wrapped around the bimetal bar 5l' and carries a relatively small current, so that .it will require from four to six hours before the temperature of particularly the bimetal bar 57 will be raised to a value on the order of 250 F., with resultant snapacting movement of bimetal bar 59 to its other operative closed position.

I provide further an electromagnetic switch designated by numeral 73, said switch comprising a pair of xed contact members 'i5 and Tl, a contact bridging member 79, which is insulatedly connected with and movable by an electromagnetic armature core 8|, which armature core is adapted to be energized by a coil 83. Electromagnetic switches of this kind are old and well known in the art.

I provide further a time-controlled switch designated generally by the numeral 85, and while I have shown two contact or switch arms 8l and 89 only, I desire it to be clearly understood that I provide in addition thereto, any time-telling device, which may-bea mechanical clock or which `may be an electric motor operative continuously switch 25.

aseaire tion of heating coil li is controlled jointly by the lower thermal switch 25 and the time-controlled switch 85.

I provide a pair of supply circuit conductors @l and 93, contact arm 0l being connected to supply circuit conductor 9|. I provide a conductor 95 connecting arm 31 to contact terminal l5, while contact-terminal l1 is connected by a conductor 3l with a conductor 99, connecting one terminal of heater 2| to the two terminals 5| and t5 of the two thermal switches of the thermal retarder unit. The second contact 53 of the rst thermal switch is connected by a conductor |0| to contact arm 0l and to the ilrst supply circuit conductor 9 A conductor |03 connects contact arm 89 of the time-controlled switch with contact Sl of the second thermal switch, which contact 67 is connected by a conductor E05 with one terminal of heating coil il, the other terminal of which is connected by a conductor ll with the rigid Contact arm 35 of the lower thermally-actuable 'Ihe other terminal of heater 2| is connected by conductor |09 with contact arm 35. A conductor l|| connects contact arm 33 with one terminal of coil 83, the other terminal of which is connected by a conductor H3 with the second supply circuit conductor 93.

The general object of my invention is to provide a control system that shall be effective to ensure that in case only a relatively small quantity of hot water has been withdrawn from the tank during a twenty-four hour day, energization of the heater will be delayed until after a. predetermined time delay period after closing of the time clock; but that if a relatively large quantity of hot water is withdrawn from the tank, energization of the heater will be effected as soon as the thermal retarder unit is subject to cold water, irrespective of the position of the time-controlled switch, said energization continuing until substantially all of the water in the tank is hot.

Let it be assumed that only a relatively small quantity of cold water is in -the tank during an on-peak period, suicient to subject tube 2l to cold water, but not enough to subject the thermal retarder unit 39 to cold water. Under these conditions the switch arms 33 and 35 will be in engaged position; the rst thermal switch, in'c'luding bimetal bar dl, will be in open position; the time-controlled switch will also be in open position; and the second thermal switch of the thermal retarder unit will also be in open position. Energization of the heating coil il will be effected upon closure of the time-controlled switch 85 through a circuit substantially as follows: from the supply circuit conductor 9|, through closed switch 85, through conductors |03 and |05, heating coil 7|, conductor lill, through closed switch 25, conductor coil B3, and through conductor H3 to the second supply circuit conductor 93. This will cause temperature rise of the second thermal switch, which will reach its predetermined maximum value on the order of 250 F., with resultant snap-acting movement of the contact bridging member 63 into position to engage contact terminals 65 and 6l, after a time delay period on the order of four to six hours. When closure of the second thermal switch occurs, the following energizing circuit through heater 2| is effected: from the rst supply circuit conductor 9|, through closed switch 85, conductor |03, through closed second thermal switch of the thermal retarder unit comprising contacts 5l, 55, and contact bridging member 53, then throu'gh conductor 9S, heater 2|, conductor |09, closed switch 25, through conductor coil 83, and through conductor I I3 to the second supply circuit conductor 93. The energized coil 83 causes upward movement of the core 8| and attendant engagement of the contacts 15 and 11 by contact bridging member 19, thereby closing another energizing circuit through heater 2|, which is independent of the time-controlled switch 85. This circuit is as follows: from the first supply circuit conductor 9|, through conductor 95, through the engaged contact members 15 and 11 and contact bridging member 19, through conductors 91 and 99, heater 2|, conductor |09, closed switch 25, conductor I||, coil 83, and conductor ||3 to the second supply circuit conductor 93. Irrespective now of whether the time-controlled switch 85 is closed or is open, energization of the heater 2| will continue until substantially all of the water in the tank is hot, when opening of the lower thermally-actuable switch 25 will occur with attendant deenergization of heater 2|. This operation may be caused by extraordinary demands for hot water during late night hours before all of the cold water in the lower part of the tank has been heated to a predetermined high temperature.

Let it now be Iassumed that suicient hot water was withdrawn from the tank to subject not only the lower thermally-actuable switch 25 to cold water, but also the thermal retarder unit 39. This will cause immediate reclosure of the first thermal switch, comprising bimetal bars 4I and 45, to the position shown in full lines in Fig. 2, resulting in closing an energizing circuit for heater 2| traceable as follows: from first supply circuit conductor 9| through conductor |0|, through the engaged contact members 53 and 5| with contact bridging member 49, through conductor 99, heater 2|, conductor |09, closed switch 25, ccnductor I||, coil 83, and conductor H3, to the second supply circuit conductor 93. The energized coil 83 will cause upward movement of armature core 8| and of contact bridging member 19, with the result that the hereinbefore described energizing circuit, through heater 2| and through the electromagnetic switch contacts and contact bridging member, will be effected, which will permit of continuing energization of the heater 2 I, irrespective of the position of the timecontrolled switch, until substantially all the water in the tank is hot, when tube 21 will be subject to hot water with resultant disengagement of contact arms 33 and 35 yand attendant deenergization of the heater 2| and of the coil 83 of the electromagnetic switch.

Referring now to Figs. 3 and 4 of the drawings, I have there shown a similar domestic hot water tank, as shown in Fig. 1, having mounted thereon ,a lower thermally-actuable heater control switch 25, but having a tarder switch |I4 lation to the tank thereof.

This thermal retarder heater control unit includes a first thermal switch, substantially the same as hereinbefore described in connection with Fig. 2 of the drawings, as well as a second thermal switch comprising`bimetal bars 51 and 59, the latter bimetal bar having insulatedly mounted thereon a contact bridging member B3. In order to simplify the explanation of the control' embodying this form of thermal retarder, the elements oi the first and of the second thermal switch have been given the same numerals as used in Fig. 2 of the drawings.

different form of thermal remounted in heat-receiving reat substantially the mld-height I provide a third thermal switch, constituting a part of the thermal retarder unit andincluding a first bimetal bar ||5 having one end fixed and having pivot plates ||1 thereon at its other forked end which are engaged by the pointed ends of a second bimetal bar |9, both of said bars having at their adjacent ends a slot therein, in which slot is positioned an over center spring |2I, the ends of which are suitably secured tothe bimetal bars ||5 and I|9 beyond the ends of the respective slots to cause snap-acting movement of the free end of bar 9 in response to temperature changes. A contact bridging member |23 is lnsulatedly mounted on the outer end of bimetal bar ||9 and is adapted to engage with and be disengaged from a cooperating pair of fixed contact members |25 and |21 witi a snap action. When contact bridging member |23 is out of engagement with the fixed contacts |25 and |21, bar 9 is adapted to be in operative engagement with a stop member |29. The bimetal bar |I5 has insulatedly mounted thereon a heating coil |3| which is adapted to carry the same current as traverses the heater 2| and is adapted to raise the temperature of the bimetal bar ||5 to a value on the order of 250 F. to cause the normally openswitch to be moved with a snap action to closed position. It may be here repeated that 1 the first thermal switch, including bimetal bars 4| and 45, is adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot water in the tank, and that the second thermal switch, including bars 51 and 59, is adapted to be in open position when subject to cold and to hot water in the tank, but to be moved with a snap action to closed position when subject to a temperature on the order of 250 F.

'I'he first supply circuit conductor 9| is connected to the rst resilient contact arm 81 of the time-controlled switch and is connected by conductor |0| with terminal 53 of the first thermal switch, which terminal is connected by a conductor |33 to the terminal |21 of the third thermal switch of the thermal retarder unit. The two contacts 5I and I 25 of the'iirst and third thermal switches are connected by a conductor |35, which conductor is connected by a conductor |31 to one terminal of heating coil |3|, the other terminal of which is connected by a conductor |39 to one terminal of heater 2|, the other temilnal of which is connected by a conductor |09 with substantially rigidcontact arm 35. The other contact arm. 33 ofthe lower thermally-,actuable switch 25 is connected by a conductor |4| with the second supply circuit conductor 83. The second contact arm 89 of the time-controlled switch is connected by conductor |03 with contact -51 of the second thermal switch, which contact is connected by a conductor |05 with one terminal of heating coil 1| on bimetal bar 51, the other terminal of which is connected by a conductor |01 with the rigid contact arm 35 of the lower thermally-actuable switch 25. 'I'he first terminal of heating coil |3| of the third thermal switch of the thermal retarder unit and conductor |31 are connected by a conductor |43 with contact 85 of the second thermal switch of the thermal retarder unit. Y

Let it now be assumed that sufficient hot water has been withdrawn from the tank to subject not only the lower thermally-,actuable switch 25 t0 cold water, but also to subject the thermal retard. er switch unit to cold water, with the result that the rst thermal switch, comprising bimetal bars 4I and 45, has been moved with a snap action into the position shown in full lines in Fig. 4 of the drawings, that is contact bridging member 1.9 is

in engagement with contacts 5| and 53. In this case, an energizing circuit will be closed through heater 2 I, traceable as follows: from the rst supply circuit conductor 9| through conductor |0I, through the Closed switch of the rst thermal switch, comprising contact bridging member 49 and contacts 5| and 53, through conductor |31. through heating coil |3|, conductor |39, heater` 2|, conductor |09, through the closed switch 25, and through conductor i4! to the second supply circuit conductor 93. Energ-ization of heater 2| through this circuit will continue for a relatively short period of time, say on the order of fteen to thirty minutes, that is until vthe energized coil |3| has caused an increase in the temperature of bimetal bar I5 to a value on the order of say 250 F., when snap-acting movement of contact bridging member |23. into engagement with contacts |25 and |21 will take place, causing a new circuit to be closed through heater 2|, traceable as follows: from supply circuit conductor 9|, con ductors and |33, through the engaged Contact members |25 and |21'with contact bridging member |23, through conductor |31, coill i3I, conductor |39, heater 2|, conductor |09, engaged contact arms 35 and 33, and through conductor lili to the second supply circuit conductor 93. Within a relatively short time, which may be on the order of one or two hours, depending upon the amount of cold water in the tank above the thermal retarder switch unit, and the raising of the temperature of this quantity of cold water to a value on the order of 150 F., with resultant opening movement of the iir'st thermal switch of the thermal retarder switch unit, with attendant disengagementof Contact bridging member 49 from contacts and 53 will be effected. When thisV happens, the energization of heater 2i will continus through the last above described circuit, which energization will continue until substantially all of thek water in the tank is hot, irrespective of the position of the time-cntrolled switch B5, with attendant subjecting of the lower thermally-actuable heater control switch 25 to hot water and opening movement of the switch controlled by the expansion rod 29 and attendant deenergization of heater 2| when substantially all of the water in the tank is hot. It will be noted that the operation of the third thermal retarder switch hereinbefore described is substantially the same as that of the electromagnetic switch shown in Fig. 2 o1 the i drawings.

Let it now be assumed that only a relatively small quantity of hot water has been withdrawn from the tank, the quantity being such as to cause entrance of cold water into thetank only suiilcient to subject the lower thermally-actuable switch 25 to cold Water. When Ithis happens` at any time during ori-peak hours, that is with the time-controlled switch 85 in open. position, no energization of heater 2| can be effected, but as soon as closure of the time-controlled switch 35 occurs, an energizing circuit Will be closed through heating coil 1l of the second thermal switch comprising the blmetal bars 51 and 59, this circuit being traceable as follows: from the first l supply circuit conductor 9 through contact arms 81 and 89 which are now engaged, through conductors |03 Aand |05, heating coil 1|, conductor |01, throughlclosed switch 25, and through conductor Mi to the second supply circuit conductor 93. After a time period of delay on the order of four to six hours, the second thermal switch will have had its temperature raised to Ya valve which will cause snap-acting movement of the contact bridging member B3 into engagement with contacts and 61, whereby an energizing circuit through Vheater 2| is effected, traceable as follows: from the first supply circuit lconductor 9|, closed switch 85, conductor |03, engaged contact members 65 and B1 and contact bridging,

member 63, conductor |43, heating coil |3| of the third thermal switch, conductor |39, heater 2|, conductor |09, closed switch 25, and through conductor |4I to the second supply circuit conductor 93. This energizing circuit will continue for a. relatively short period lof time until the heating coil |3| of the third thermal switch has raised the temperature thereof to a value sumcient to cause snap-actingmovement of contact bridging member |23 into engagement with contacts |25 and |21, when a new energizing circuit through heater 2| is closed as follows: from vthe first supply circuit conductor 9|, through conductors I0| and |33, through contacts |21 and |25 and contact bridging member |23, through conductors |35 and |31, heating coil |3|, conductor |39, heater 2|, conductor |09, closed switch 25, conductorvldI, to the second supply circuit conductor 93. Energization of the heater 2| will therefore continue, irrespective of the position of the time-controlled switch 85, until substantially all of the water in the tank is hot, when opening of the lower thermally-actuable switch be effected in case of extraordinary demands for hot water during late night hours before all of the cold water in the lower part of the tank` has been heated to a predetermined high tempera/ture.

It may be here pointed out that the two forms n of control systems shown in Figs. 2 and 4 of the drawings are such as to ensure that a full tank of hot watervwill be provided at the start of daylight hours, even though extraordinary demands for het water are made upon the tank during the late night hours, as in case of sudden illness among the members of the family. In case relatively large withdrawals of hot water occur during the onpeakhours or periods of a twenty-four hour day, immediate energization of the heater will take place, which energization will continue until substantially all of the water in the tank is hot.

Variousmodifications may be made in the systems embodying my invention without departing from the spirit and scope thereof, and all such modifications coming clearly within the scope of the appended claims are to be considered as cov-4 ered thereby.

I claim as my invention:

. l. A control system for a hot Water tank hav-v ing an electric heater, comprising a lowerther.

mally-actuable heater control switch located on the tank adjacent the lower end thereof, a timeclosedv position when heated `to an appreciably higher temperature, and a heating coil for said second thermal switch, a third switching means connected in shunt with said time. switch and said second thermal switch, and other electric connections between said switches and said electric heater to cause immediate energization of said heater in case said thermal retarder unit is subject to cold water in the tank, irrespective ofthe position of the time-controlled switch.

2. A control system for a hot water tank having an electric heater, comprising a lower thermally-actuable heater control switch located on the tank adjacent the lower end thereof, a timecontrolled switch adapted to be in open position during on-peak periods of the day, a thermal retarder heater control switch unit mounted on the tank intermediate the ends thereof and including a first v thermal switch adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot water in the tank, and a second thermal switch adapted to be in open position when subject to hot water in the tank and to be in closed position when heated to an appreciably higher temperature, and a heating coil for said second thermal switch, a third switching means con- 'nected in shunt with said time switch and said second thermal switch, and other electric connections between said switches and said electric heater to cause immediate energization of sain heater in case said thermal retarder unit is subject to cold water in the tank, irrespective of the position of the time-controlled switch, said lower thermally-actuable switch being effective to cause deenergization of said heater when substantially all of the water in the tank is hot.

3. A control system for a hot water tank having an electric heater, comprising a lower thermally-actuatable heater control switch located on the tank adjacent the lower end thereof, a time-controlled switch adapted to be in open position during on-peak periods of the day, athermal retarder heater control switch unit mounted on the tank intermediate the ends thereof and including a first thermal switch adapted to be in closed position when subject to cold 'water in the tank and to be in open position when subject to hot water in the tank, and a second thermal switch adapted to be in open position when subject to hot water in the tank and to be in closed position when heated to an appreciably higher temperature, and a. heating coil for said second thermal switch, a third twitching means connected in shunt with said time switch and said second thermal switch, and other electric connections between said switches and said electric heater tol, cause immediate energization of said heater through said lower thermally-actuable switch and said first. thermal switch of the thermal retarder unit and continuing through said third switching means and said lower thermallyactuable switch until substantially all of the 4water in the tank is hot.

4; A control system for a domestic hot water tank having an electric heater, subject to withdrawal of varying quantities of hot water during;r a twenty-four hour day, comprising a lower therI mally-actuable heater control switch positioned in heat-receiving relation on the tank adjacent to the lower end thereof and adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot Water in the tank, a time-controlled switch adapted to be in closed position during off peak periods only, a thermal retarder hearts control switch mit mounted in heat-receiving relation on the tank intermediate the ends therer of and including a first thermal switch adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot water in the tank, a second thermal switch adapted to be in open position when subject to cold and to hot water in the tank and to be in closed position when heated to an appreciably higher temperature and a heats ing coil for said second thermal switch, the en-` ergization of which is controlled jointly by said time-controlled and said lower thermally-actu-- able switch, a normally-open third switching means connected in shunt with said time-controlled and said second thermal switch and adapted to be moved into closed position by the current traversing said heater and other connections between said switches and said heater to cause immediate energization of said heater in case said thermal retarder is subject tof cold water in the tank, irrespective of the position of the time-controlled switch, and to prevent; energization of said heater for a predetermined length of time after closing of said time-controlled switch in case only the loWer'thermally-actuable switch is subject to cold water.

5. A control system for a domestic hot water tank having an electric heater, subject to with.n drawal of varying quantities of hot water during a twenty-four hour day, comprising a lower einer mally-actuable heater control switch positioned in heat-receiving relation on the tank adjacent to the lower end thereof and adapted to be in closed position when subject to colf. water in the tankand to be in open position when subject to hot water in the tank, a time-controlled switch adapted to be in closed position during oit-peak periods only, a thermal retarder heater control switch unit mounted in heat-receiving relation on the tank intermediate the ends thereof and including a rst thermal switch adapted to be in closed position whenl subject to cold water in the tank and to be in open position when subject to hot water in the tank, a second thermal switch adapted to be in open position when subject to cold and to hot water in the tank and to be in closed position when heated to an appreciably higher temperature and a heating coil for said second thermal switch, the energization of which is controlled jointly by said time-controlled and said lower thermally-actuable switch, a normally-open third switching means connected in shunt with said time-controlled and said second-A thermal switch and adapted to be moved ini-o Aclosed position by the current traversing said drawal of varying quantities of hot water during a twenty-four hour day, comprising a lower thermally-actuable heater control switch positioned in heat-receiving relation on the tank adjacent to the lower end thereof and adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot water in the tank, a time-controlled switch adapted to be in closed position during-oii-peak periods only, a thermal retarder heater control switch unit mounted in heat-receiving relation on the tank intermediate the ends thereof and including a rst thermal switch adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot water in the tank, a second thermal switch adapted to be in open position when subject to cold and to hot water in the tank and to be in closed position when heated to an appreciably higher temperature and a heating coil for said second thermal switch, the energization of which is controlled jointly by said time-controlled and said lower thermally-actuable switch, a normally-open third switching means connected in shunt with said time-controlled and said second thermal switch and adapted to be moved into closed position by the current traversing said heater and other connections between said` switches and said heater to cause immediate energization of said heater in case said thermal retarder is subject to cold water inthe tank, irrespective of the position of the time-controlled switch and to cause energization of said heater after a predetermined time period of delay after closing of said time-controlled switch in case only the lower thermally-actuable switch is subject to cold water, deenergization of said heater being eiected by said lower thermally-actuable switch when substantially al1 of the water in the tank is hot.

7. A control system for a hot water tank having an electric heater, comprising a lower thermally-actuable heater fontrol switch located on the tank adjacent the lower end thereof, a timecontrolled switch, a thermal retarder switch means mounted on the tank intermediate the ends thereof and including a first thermal switch adapted to be in closed position when subject to cold water and to be in open position when subject to hot water, a second i thermal switch adapted to be in open position when subject to hot water and to be in closed position owhen heated to an appreciably higher temperature, and a heating coil for said second thermal switch, an electromagnetic switch connected in shunt circuit with said time-controlled switch and said second thermal switch and other electric connections between said switches and said heater to cause immediate energization of said heater in case oi said thermal retarder switch means being sub- J'ect to cold water in the tank, irrespective oi Y the position of the time-controlled switch.

8. A control system for a hot water tank having an electric heater, comprising a lower thermally-actuable heater control switch located on the tank adjacent the lower end thereof, a timecontrolled switch, a thermal retarder switch means mounted on the tank intermediate the ends thereof and including a first thermal switch adapted to be in closed position when'subject to cold water and to be in open position when subject to hot water, a second thermal switch adapted to be in open position when subject to hot water and to be in closed position when heated to an appreciably higher temperature, and a heating coil for said second thermal switch, an electromagnetic switch connected in shunt circuit with said time-controlled switch and said second thermal switch and other electric connections between said switches and said heater to cause immediate energization of said heater in case of said thermal retarder switch means being subject to cold water in the tank, irrespective of the position of the time-controlled switch, said lower tl'iermally-actuable switch being effective to cause deenergization of the heater when substantially all of the water in the tank is hot.

9. A control system for a domestic hot water tank having an electric heater, subject to withdrawal of varying quantities of hot water during a twenty-four hour day, comprising a lower thermally-actuable heater control switch positioned in heat-receiving relation on the tank adjacent to the lower end thereof and adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot water in the tank, a time-controlled switch adapted to be in closed position during oil-peak periods only, a thermal retarder heater.control switch unit mounted in heat-receiving relation on the tank intermediate the ends thereof and including a first thermal switch adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot water in the tank, a second thermal switch adapted to be in open position when subject to cold and to hot water in the tank and to be in closed position when heated to an appreciably higher temperature and a heating coil for `said second thermal switch, the energization of which is controlled jointly by said time-controlled and said lower thenmally-actuable switch, a normallyopen third switching means connected in shunt with said time-controlled and said second thermal switch and adapted to be moved into closed position by the current traversing said heater and other connections between said switches and said heater to cause immediate energization of said heater through said lower thermally-actuable switch and said rst thermal switch in case said thermal retarder is subject to cold water in the tank, irrespective of the position of the time-controlled switch, and to continue energization of said heater through said lower thermally-actuable switch and said third switching means until substantially all of the water in the tank is hot.

10. A control system for a domestic hot water tank having an electric heater, subject to withperiods only, a thermal retarder heater control switch unit mounted in heat-receiving relation on the tank intermediate the ends thereof and including a rst thermal switch adapted to be in closed position when subject to cold Water in the tank and to be in open position when subject to hot water in the tank, a second thermal switch adapted to be in open position when subject to cold and to hot water in the tank and to be in closed position when heatedto an appreciably higher temperature and a heating coil for said second thermal switch, the energization of which Y is controlled jointly by said time-controlled and said lower thermally-actuable switch, a normallyv heater to cause energlzation of said heater after a predetermined time period of delay after closure of said time-controlled switch through said lower thermally-actuable switch, said second thermal switch and said time-controlled switch in case only the lower thermally-actuable switch is subject to cold water at the time of closure of the time-controlled switch, said energization continuing through said lower thermally-actuable switch and said third switching means until substantially all of the water in the tank is hot, when deenergization of the heater is effected by said lower thermally-actuable switch.

11. A control system for a domestic hot water tank having an electric heater, subject to with-- drawal of varying quantities of hot water during a twenty-f our hour day. comprising a lower thermally-actuable heater control switch positioned in heat-receiving relation on the tank adjacent to the lower, end thereof and adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot water in the tank, a time-controlled switch adapted to be in closed position only during oil?- peak periods, a thermal retarder heater control switch unit mounted in heat-receiving relation on the tank intermediate the ends thereof and including a rst bimetal snap-acting switch adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot water in the tank, a second bimetal snap-acting switch adapted to be in open position when subject to cold and to hot water in the tank and to be in closed position when heated to an appreciably higher temperature` a heating coil for said second switch, the energization of which is controlled jointly by said lower thermally-actuable switch and said time-controlled switch. a third birnetal snap-acting switch adapted to be in open position when subject to cold and to hot water in the tank and to be in closed position when heated to a substantially higher temperature and' connected in shunt circuit with said time-controlled 'and said second thermal switch, and a heating coil for said third switch energized by the heater current and other connections between said switches and said heater to cause immediate energization of said heater through said lower thermally-actuable switch and said rst snap-acting switch in case said thermal retarder switch unit is subject to cold water and to continue said lenergization through said lower thermally-actuable switch and said third snap-acting switch until substantially all of the Water in the tank is hot.

12. A control system for a domestic hot water tank having an electric heater, subject to with drawal of varying 1quantities of hot water during a twenty-four hour day, comprising a lower thermally-actuable heater control switch positioned in heat-receiving relation on the tank adjacent to the lower end thereof and adapted to be in closed position when subject to cold water in the tank and to be in open position when subject to hot water in tank, a time-controlled switch adapted to be in closed position only during oipeak periods, a thermal retarder heater control switch unit mounted in heat-receiving relation on the tank intermediate the ends thereof and iucluding a first bimetal snap-acting switch adapted to be in closed position when subject to cold water in the tank andto be in open position when subject to hot water in the tank, a second bimetal snap-acting switch adapted to be in open position when subject to cold and to hot water in the tank and to be in closed position when heated to an appreciably higher temperature, a heating coil for said second switch, the energization of which. is controlled jointly by said lower thermally-actuable switch and said time-controlled switch, a third bimetal snap-acting switch adapted to be in open position when subject to cold and to hot water in the tank and to he in closed position when heated to a substantially higher temperature and connected in shunt circuit with said time-controlled and said second thermal switch, and a heating coil .for said third switch energized by the heater current othe.` connections between said switches and said heater to cause immediate energization of said heater through said lower thermally-actuable switch and said rst snap-acting switch in case said thermal retarder switch unit is subject to cold water and to continue said energization through said. lower thermally-actuable switch and sale third snap-acting switch vuntil substantially all of the water in the tank is hot, and to cause energization of said heater with a predetermined time delay after closure of the time-controlled switch, through said lower thermally-actuable switch, said second snap-acting switch and said time-controlled switch in case only said lower thermally-actuable switch is subject to cold water and to continue said energization through said lower thermally-actuable switch and said third snap-acting switch until substantially all of the water in the tank is hot when deenergization of the heater i5 effected by said lower thermallyactuable switch.

CLARK M. OS'IFRHELD 

